1. Field Of The Invention
The present invention relates to blends of ethylene vinyl alcohol and finely divided platelet-type mica particles. These compositions are exceptionally effective as a thin barrier layer in multilayer packaging constructions such as bottles, jars, cups, trays and other rigid containers.
2. Prior Art
In recent years major changes have been occurring in the packaging industry as plastic containers have been replacing those made from metal and glass, as well as going into innovative new types of containers. This expansion in the use of plastics has been supported by the development of high barrier resins which are effective in thin layers in reducing the penetration of oxygen as well as flavors and odors during the packaging, transportation and storage of sensitive foods, drugs and chemicals.
The plastic resins commonly used in the packaging industry can be divided into three general classifications based on their permeability to oxygen. The oxygen permeability value (OPV) of polymer films is generally expressed in cubic centimeters per day for a film area of 100 square inches (645 cm.sup.2) at a pressure gradient of one atmosphere O.sub.2 per mil of film thickness.
The oxygen barrier of many films vary with temperature and relative humidity. Generally oxygen permeability increases with temperature and relative humidity. Since most food products have a high water content or are subject to storage under hot humid conditions, the OPV values disclosed herein are all measured at 80% RH and 30.degree. C. unless reported otherwise.
The high oxygen barrier plastics generally have an OPV of less than 1. This group consists of polyvinylidene chloride copolymers (PVDC), acrylonitrile copolymers (PAN) and ethylene vinyl alcohol copolymers (EVOH). Copolymers of ethylene and vinyl alcohol suitable for use in the present invention can be prepared by the methods disclosed in U.S. Pat. Nos. 3,510,464; 3,560,461; 3,847,845; and 3,585,177. The medium barrier resins generally have an OPV in the range of 1 to 10. These include plastics like nylon 6, nylon 6--6, oriented polyethylene terephthalate (OPET), and rigid polyvinyl chloride (PVC). Low barrier packaging plastics have an OPV of greater than 10 to over 300. These include high density polyethylene (HDPE), polypropylene (PP), polybutylene terephthalate (PBT), and polystyrene (PS).
The use of platelet shaped inorganic fillers to improve the barrier properties of a resinous film is very old technology. These blends of fillers and resins function by forming so-called tortuous paths for diffusion of permeating species through the composite film. The use of aluminum flake powder in paint films to improve moisture barrier and weathering has been utilized since at least the 1890's.
There are a number of examples in the patent literature on the use of inorganic platelet type fillers to improve the barrier properties of low and medium barrier polymers. The use of mica in polyethylene is disclosed in U.S. Pat. No. 3,463,350 to improve the oxygen barrier of rigid molded food containers. U.S. Pat. No. 4,536,425 describes a technique for blending relatively coarse mica plates with a polar thermoplastic, especially PET, and compounding with shear to delaminate the mica platelets.
British Pat. No. 1,136,350 describes the use of platelet type fillers in a variety of low barrier plastics including polyethylene, polypropylene, polystyrene or ethylene copolymers containing at least 50 mole percent of ethylene units. The films are claimed as useful as packaging materials for food, e.g., cereals and potato chips.
U.S. Pat. Nos. 4,528,235 and U.S. 4,618,528 disclose thin polymer films containing small sized platelet type filler. The polymer has a melt flow rate of from 0.01 to 10 grams per 10 minutes at 190.degree. C. to 250.degree. C. as measured on ASTM Test No. D-1238 at a load of 1000 to 2160 grams. The preferred polymer is polycaprolactam or high density polyethylene. Ethylene/vinyl alcohol copolymers are also disclosed as being suitable polymers. There is from 10 to 50 percent of a platelet filler having an average equivalent diameter of from 1 to 8 micrometers, a maximum equivalent diameter of about 25 micrometers, and an average thickness of less than 0.5 micrometers. The preferred filler is talc. Ground mica, platelet silicas, flaked metal and flaked glass are also disclosed as being suitable fillers. The film is from 10 to 100 micrometers thick and can be a single ply or a component of a laminate. The use of ethylene/vinyl alcohol copolymers in combination with mica having a high aspect ratio is not disclosed.
The use of platelet type fillers to reduce the permeability of low and medium barrier polymers has been shown to yield a 50% to 300% improvement in barrier properties. However, resulting barrier compositions are still too high in OPV to be used as packaging films or layers of a laminate for the more oxygen sensitive foods, drugs and chemicals now packaged in glass and metal.
The extensive prior art shows the unfilled need for a specific combination of an inexpensive inorganic filler and a high barrier resin which would have a substantially improved barrier and which could be processed on conventional polymer processing and container forming equipment to yield attractive, transparent, packaging with long shelf life.